Built to accommodate broad gauge trains, the South Wales Mineral Railway served the colliery of the Glyncorrwg Coal Company. Authorised in 1853, the section between Briton Ferry and Tonmawr opened on 1st September 1861 with an extension to Glyncorrwyg coming on line on 10th March 1863. A standard gauge conversion occurred in May 1872.

The route from Tonmawr cut through the hill towards Cymmer via a single bore tunnel measuring 1,109 yards. Though largely unlined, its roof boasts an irregular brick arch throughout and repair collars have been installed over the years in areas of weakness. Originally contracted to cut it was J G McKenzie and Berwick-born engineer John Dickson but his contract was re-let after three years due to a lack of progress.

Death attended on 16th August 1902. Trains traversing the route in opposite directions had two opportunities to pass - at Tonmawr and Cymmer, either end of the tunnel - although the former was only rarely used. However, on this day, the colliery company's cashier was in a hurry to get to Britton Ferry so asked the Traffic and Telegraph Clerk at Glyncorrwyg if his train, travelling in the Up direction, could pass the Down service at Tonmawr, so speeding his journey. With the driver thus instructed, the train departed.

The Clerk wired Cymmer to say that the train had set off but no reply came. He then wired Incline Top - an outpost some distance beyond the tunnel - to discover the location of the Down engine. It had already left. He wired again, asking them to call Tonmawr and arrange for the train to be stopped there. But the signalman had left his post early.

A collision was inevitable, the coming together occuring close to the tunnel's centre. Climbing a 1:70 gradient, driver Hughes saw the Up train approaching and brought his Down engine to a stop just as it was struck.

Although the line only officially carried minerals, passengers were often accommodated in the guard's van. On this occasion, seven were on board - all sustained serious injuries, two lost their lives. The cashier, riding on the footplate, was also injured together with both drivers and firemen.

The end of the line came on 13th July 1947 when a landslip blocked the approach cutting at the tunnel's western portal. Since its abandonment, this end of the bore has become flooded, with the waters first making their presence felt close to the centre. Given the gradient, it's likely that the tunnel is drowned to its roof for about 100 yards.